The present invention relates to a novel polyester resin having an excellent heat resistance and a process for the preparation thereof, and more particularly to an aromatic polyestersulfone represented by the formula: ##STR3## wherein R and R' are the same or different and each is an alkyl or alkoxyl group having 1 to 4 carbon atoms, Ar is a bivalent aromatic group, x and y are 0 or an integer of 1 to 4, but the sum of x and y is from 1 to 8, m is a positive integer and n is 0 or a positive integer.
Aromatic polyesters consisting of aromatic dicarboxylic acids and bivalent phenol compounds have been known for many years. Also, an interfacial polycondensation process, a solution polymerization process, a melt polymerization process and so on have been known as processes for preparing such polyasters. The aromatic polyesters have excellent heat resistance and mechanical property, and are one of useful substances as molding materials. In general, however, the aromatic polyesters have the property that the ester linkage is hydrolyzed, and particularly the alkali resistance is poor. Further, although the aromatic polyesters have an excellent heat resistance, the heat resistance is not always satisfactory in the application to the electric and electronic fields. For instance, the heat resistance in molten solder is not sufficient. Therefore, it has been required to further raise the usable temperature region of the aromatic polyesters.
For this purpose, many attempts have been made with respect to polyesters of 2,2-bis(4-hydroxyphenyl)propane and terephthaloyl or isophthaloyl chloride or a derivative thereof. The polyesters of this type put on the market are melt-processable, and moreover the moldings obtained therefrom are transparent and have an excellent machanical property. However, the glass transition temperature of these polyesters is about 240.degree. C. (obtained from logarithmic decrement measured by a dynamic viscoelasticity spectrometer) and the heat distortion temperature of the moldings is below 180.degree. C. and, therefore, it has been difficult to use the polyesters in applications requiring heat stability at high temperatures.
It is known that aromatic polyesters obtained by using bis(4-hydroxyphenyl)sulfone (hereinafter referred to as "bisphenol S") as a bisphenol component instead of 2,2-bis(4-hydroxyphenyl)propane have a high glass transition temperature. For instance, a copolymer of bisphenol S and a mixture of terephthaloyl chloride and isophthaloyl chloride in a ratio of 1/1 by weight has a glass transition temperature of 295.degree. C. However, the polyesters obtained from bisphenol S have the drawbacks that the processability in melt molding is very low, that the ester linkage is very easy to be hydrolyzed and the alkali resistance is particularly low, and also that the cast films thereof are fragile and opaque.